Exhaust controls of carbon dioxide gas and the like are being tightened up against a background of an enhancement of the environmental movement. In the car industry, not only automobiles using fossil fuels such as gasoline, diesel oil, and natural gas, but also EVs and HEVs have been developed actively. In addition, a recent sudden rise in prices of the fossil fuels has accelerated the development of EVs and HEVs. In addition, in the field of batteries for the EVs and the HEVs, nonaqueous electrolyte secondary batteries represented by a lithium ion secondary battery with higher energy density than other batteries are noticed, and the percentage of the nonaqueous electrolyte secondary battery has been increasing greatly.
On the other hand, the batteries for EVs and HEVs are required to achieve a highly developed traveling performance as a basic performance of automobiles as well as the environmental accommodation. In order to achieve the highly developed traveling performance, not only increasing a battery capacity for enabling the automobiles to travel long distance but also increasing a battery output power having an effect on acceleration performance or hill climbing performance of the automobiles, that is, improving a rapid discharge characteristic is needed.
In addition, in order to inhibit total energy consumption of EVs or HEVs, being able to collect a generated electric power at the time of deceleration by using an electric brake is needed, that is, in order to improve an input characteristic, improving a rapid charge characteristic of the battery is also needed. This is because, since actual driving of the automobiles has not only acceleration zones but also deceleration zones frequently, control of total energy consumption of EVs or HEVs depends on how much electric energy can be collected in the deceleration zone.
When such rapid discharging or charging is performed, a high current is applied in the battery, so that the battery internal resistance greatly affects battery characteristics. Especially, in the batteries for EVs or HEVs, in order to obtain the output/input characteristics sufficiently, even if a state of charge varies, a low and constant internal resistance is required. As for the internal resistance due to the variation of the state of charge, voltage is measured when a battery is charged or discharged at several points of current values for a certain period of time, and a slope of the voltage with respect to the current value is calculated to give an IV resistance value as the internal resistance. The IV resistance value is an index showing how much current can be applied to a battery.
Here, an example of specific structures of nonaqueous electrolyte secondary batteries 10 utilized for these EVs or HEVs will be described using FIGS. 2 to 6. FIG. 2 is a perspective view of a cylindrical nonaqueous electrolyte secondary battery. FIG. 3 is an exploded perspective view of a rolled electrode in the cylindrical nonaqueous electrolyte secondary battery in FIG. 2. FIG. 4 is a perspective view of the collector plate shown in FIG. 3.
FIG. 5 is a partially broken perspective view showing a state before the collector plate is pressed to the rolled electrode. Furthermore, FIG. 6 is a partially broken elevation view showing a state where the collector plate is pressed to the rolled electrode and a laser beam is irradiated.
As shown in FIG. 2, respective covers 12 are welded at both ends of a cylinder 11 to form a cylindrical shaped battery outer can 13, and a rolled electrode 20 as shown in FIG. 3 is put in the battery outer can 13 to form the nonaqueous electrolyte secondary battery 10. Pair of positive and negative electrode terminal devices 14 is placed on the covers 12. The rolled electrode 20 and the electrode terminal device 14 are connected in the battery outer can 13, and electric power generated from the rolled electrode 20 can be taken out from a pair of the electrode terminal devices 14. Moreover, a pressure switching gas exhaust valve 15 is placed on each cover 12.
As shown in FIG. 3, a strip-shaped separator 23 is interposed between a continuous positive electrode plate 21 and a continuous negative electrode plate 22, and then the whole is rolled spirally to form the rolled electrode 20. The positive electrode plate 21 includes a continuous substrate 211 made of aluminum foil and a positive electrode mixture layer 212 coated with slurry containing a positive electrode active material on both sides of the substrate 211, and the negative electrode plate 22 includes a continuous substrate 221 made of copper foil and a negative electrode mixture layer 222 coated with slurry containing a carbon material as a negative electrode active material on both sides of the substrate 221. Moreover, in the separator 23, a nonaqueous electrolyte is impregnated. In the nonaqueous electrolyte secondary battery 10, in order to keep output characteristics, the positive electrode plate 21 and the negative electrode plate 22 are designed so as to be thin, and in order that a facing area of the positive electrode plate 21 and the negative electrode plate 22 becomes large, the plates 21 and 22 are designed so as to have a continuous shape.
In the positive electrode plate 21, an uncoated part not coated with the positive electrode mixture layer 212 is formed, and the uncoated part is protruded from an edge of the separator 23 to form a positive electrode substrate border 213. Similarly, in the negative electrode plate 22, an uncoated part not coated with the negative electrode mixture layer 222 is formed, and the uncoated part is protruded from an edge of the separator 23 to form a negative electrode substrate border 223. Collector plates 30 are installed on both ends of the rolled electrode 20, respectively, and these collector plates 30 are attached to the positive electrode substrate border 213 and the negative electrode substrate border 223 by laser welding or electron beam welding, respectively. A leading end of a leading part 31 protruded from an edge of the collector plate 30 is connected to the electrode terminal device 14.
As shown in FIGS. 3 and 4, the collector plate 30 includes a round shaped planar body 32, and in the planar body 32, a plurality of arcuate convex parts 33 extended radially are formed as a single-piece and are protruded to a side of the rolled electrode 20. In addition, as shown by an arrow P in FIG. 5, the collector plate 30 is pressed in a direction of the positive electrode substrate border 213 or the negative electrode substrate border 223, and then is welded by laser beam (or electron beam) irradiation as shown by a broad arrow in FIG. 6. The welding is performed by sequential spot welding while moving the laser beam in a longitudinal direction of the arcuate convex part 33, and a bottom part of the arcuate convex part 33 and the positive electrode substrate border 213 or the negative electrode substrate border 223 are welded at a welded part 34. Thus, the positive electrode plate 21 and the negative electrode plate 22 are electrically connected to the respective collector plates 30 to collect electric current.
As shown in FIG. 3, the positive electrode plate 21 or the negative electrode plate 22 of the above-mentioned nonaqueous electrolyte secondary battery is prepared in the following manner: the positive electrode mixture slurry or the negative electrode mixture slurry is coated on the continuous positive electrode substrate 211 or the continuous negative electrode substrate 221 with a predetermined thickness so as to form the uncoated part in at least one side along a long side of the substrates 211 or 221, dried, and then compressed using a compression roll so as to have a predetermined thickness, respectively. However, in the nonaqueous electrolyte secondary battery 10 for EVs or HEVs, since the positive electrode plate 21 and the negative electrode plate 22 are designed so as to be thin and to have a large facing area of the positive and negative electrode plates 21 and 22, at the time of the compression, a distortion between the positive and negative electrode plates 21 and 22 becomes large, so that a rolling gap tends to be generated when the spiral rolled electrode 20 is prepared. Since such rolling gap of the electrode plate appears especially in the positive electrode plate 21 side largely and causes a inner short circuit, it is required to reduce the rolling gap as much as possible. As for a difference in the distortion formed in the positive and negative electrode plates 21 and 22, since strong power is needed to compress metal oxide as the positive electrode active material, distortion in the positive electrode tends to be generated in comparison with the negative electrode made of a carbon material.
As for a carbon material as a conductive material in the positive electrode mixture, JP-A-7-147159 and JP-A-10-233205 disclose one using flaky graphite powder with a thickness of 1 μm or below, an average particle diameter of 1 to 50 μm, and a specific surface area of 5 to 50 m2/g, JP-A-9-27344 discloses one using a mixture of scaly graphite and fibrous graphite with a mixing weight ratio of 85:15 to 25:75, and moreover, JP-A-2000-58066 discloses one using a mixture of acetylene black, scaly graphite, and vapor-grown fibrous graphite. However, in JP-A-7-147159, JP-A-10-233205, JP-A-9-27344, and JP-A-2000-58066, it is not suggested that the above mentioned problem is generated in case of using the positive electrode plate which is formed so that the uncoated part would be formed along a long side of the continuous positive electrode substrate coated with the positive electrode mixture layer containing the positive electrode active material.
The inventors of the present invention have carried out various experiments repeatedly in order to find a condition which reduces the distortion generated at the time of compression of the positive electrode mixture layer, in the positive electrode plate which is formed so that the uncoated part would be formed along a long side of the continuous positive electrode substrate coated with the positive electrode mixture layer containing the positive electrode active material. As a result, the inventors have found out to solve the problem by an addition of a particular content of a carbon material having a particular physical property to a positive electrode mixture, and have completed the present invention.